It is generally the objective of seismic exploration to generate seismic energy, make measurements of and record the amplitude of any reflected and refracted energy at selected locations and for selected times, and then by selectively processing the recorded seismic data, to deduce the geometry of the subsurface geologic boundaries as well as some properties of the materials of the earth through which the seismic energy has propagated and from which it has been reflected.
Conventional land seismic acquisition techniques involve the use of an appropriate source (dynamite, vibrator(s), airguns(s), etc.) to generate seismic energy and a set of detectors, spread out on the surface of the earth, to detect any seismic signals due to seismic energy interacting with subsurface geologic boundaries. These detected signals are recorded as a function of time and subsequent processing of these signals, i.e., seismic "traces" or seismic data, is designed to reconstruct an appropriate image of the geologic boundaries of the subsurface and to obtain information about the subsurface materials. In simplistic terms this conventional acquisition process has a seismic wave, from a source of seismic energy, traveling down into the earth, reflecting from a particular geologic interface (i.e., a change or contrast in elastic constants, velocities, and/or densities), and returning to the surface, where the seismic wave may be detected by an appropriate detector, or detectors.
Conventionally, the detector employed to detect seismic signals on land is a geophone. A geophone is an electro-mechanical device that is coupled to the ground via an extension or "spike" that is physically inserted into the ground. This allows the geophone case to vibrate because of any earth motions, including seismic signals. Internal to the geophone case and vibrationally isolated from the case (typically by springs) is an "inertial" mass that does not vibrate with the earth. Thus, there is a small relative motion between the geophone case and its inertial mass due to any detected ground motions. This relative motion is converted to an electrical signal by having a coil of wire that moves through an electromagnetic field of a permanent magnet; the magnet may be the inertial mass with the coil attached to the geophone case, or vice versa. This electrical signal is the seismic signal that is recorded (alone, or more preferably, in combination with other electrical signals) and later processed.
Seismic field layouts vary with the exploration objective sought to be detected. However, there is usually a need to simultaneously record seismic motion at many ground positions spaced over a wide area. A seismic line usually consists of multiple detector stations with each detector station having several geophones whose output signals are combined to form a single signal (array signal) for the detector station. The geophones are arranged in an array to reject unwanted waves while enhancing the reception of desired seismic waves on the electrical signals.
The signals from these arrays (detector stations) are collected and recorded for each seismic shot (seismic energy source). Conventional land seismic data acquisition is slow, expensive and labor intensive. In particular, the activity of laying out a cable which interconnects the detector stations, so that the array electrical signals may be recorded, is time consuming and requires a substantial work force. It would be desirable to have a fast way to move the entire seismic line or portions thereof and thereby minimize survey time.
A radar system which senses seismic ground motion (vibrations) remotely is one solution for reducing the cost and time for acquiring seismic data. Two such systems are described in U.S. Pat. Nos. 5,070,483, Remote Seismic Sensing, and 5,109,362, Remote Seismic Sensing. Both patents use laser Doppler heterodyne interferometry techniques to detect the movements of the earth.
Another laser Doppler interferometry system is described in U.S. Pat. No. 4,284,350, Laser Geophone, which uses a homodyne system with a detector having side-by-side corner-cube retroreflectors for detecting vertical ground motions at a remote location. The use of a single or a side by side corner-cube retroreflector arrangement is also suggested for some detectors in embodiments of the heterodyne systems described in the before mentioned U.S. Pat. Nos. 5,070,483 and 5,109,362.
However, these systems require at least one sensing beam and return beam for each detector (geophone equivalent). An array may have as many as 32 geophones for each detector station and a seismic line may have a hundred or more detector stations. If 3-D detection is desired, there could be many additional seismic lines. The number of sensing and return beams necessary to acquire this data by a remote sensing system could be substantially reduced if only one sensing beam and return beam are used for each detector station.
In addition, some of the prior art remote sensing detectors of the before mentioned patents feature detectors which have side-by-side retroreflector configurations. In theory, the use of a side-by-side retroreflector detector requires that two approximately side-by-side sensing beams be sent to a detector where at least one of the beams is Doppler shifted (frequency modulated) by seismic motions coupled to the detector. The two modulated sensing beams may contain approximately identical Doppler shifted frequency components (common mode signals) which represent seismic motions common to the Doppler shifting optical components on the detector (assuming that both beams were Doppler shifted). Also, the modulated beams contain between them Doppler shifted frequency components (a difference signal) which represents the desired (or selected) seismic signal. When the modulated beams are combined electronically or by optical homodyning, the common mode signals cancel and the difference signal (the frequency components which are not common to both beams) remains. One problem with a homodyning system is that it is not possible to determine "up" Doppler and "down" Doppler motion from the obtained difference signal. This problem is resolved by using a heterodyning system instead. However, in spite of this problem, the difference signal provided by this prior art homodyning system does represent the vertical motions of the earth at the remote location.
The side-by-side corner-cube retroreflector configurations of the before mentioned designs, or any other type of side-by-side retroreflector design, necessarily require that two sensing beams travel different paths through the air to and from the retroreflectors. Separate air paths may have different effects upon the propagating laser sensing beams. Solar radiation heats the ground surface, causing convective air currents which break into turbulent flow. These randomly sized (roughly 1 millimeter to 1 meter) air packets have anomalous temperatures and refractive indices. The optical phase of each laser beam shifts as it passes through a region of anomalous refractive index. These air packets blow across the raypath and cause time-varying, random frequency modulation of the laser beam. Thus, two laser beams traveling through different air spaces will experience different fluctuations in each carrier frequency of the laser beams.
Since, the two laser beams are spatially separated, they will not be affected equally by the atmosphere and the atmospheric effects on the beams are not totally common mode signals. The atmospheric effects do not cancel completely when the two laser beams are combined electronically or by optical homodyning or heterodyning techniques. Thus, the difference signal will not only contain the desired Doppler signal but also an additional component which will be referred to herein as turbulent noise. Turbulent noise is especially prevalent on sunny, windy days. Consequently, if side by side retroreflectors are used as parts of the detector in a remote seismic sensing system, the presence of turbulent noise on the difference signal could prevent an accurate determination of the desired (or selected) ground motions (the desired Doppler signal).
Also, small seismic motions may not be detectable by the prior art systems. Each sensing beam is frequency modulated within the detectors when: it is incident normally on an optical component (such as a mirror, a retroreflector or a beamsplitter), it is reflected (or deflected) from its incident path by the optical component, and the optical component is moving relative to the incident path of the beam. The amount of Doppler shift at each point of reflection (deflection) is proportional to the relative velocity of the reflection point on the Doppler shifting optical component with respect to the incident path of the beam. As the beam is reflected through the detector, the Doppler shifts are cumulative; thus, the Doppler shifts add or subtract from the previous Doppler shifts on the beam. Upon exiting the detector, if the points of reflection on the combination of Doppler shifting optical components have undergone a net relative movement along the incident paths of the sensing beam, the cumulative movement of the reflection points would represent a net velocity with respect to the sensing beam. And, the sensing beam will be Doppler shifted (frequency modulated) by an amount which represents the net velocity of these reflection (deflection) points relative to the incident paths of the sensing beam. However, the seismic motion imposed upon the components which Doppler shift the sensing beam have relatively small amplitudes; thus, some seismic signals may not be detectable because of limitations, e.g., frequency drift and noise, in the homodyning or heterodyning and demodulation processes. If the amount of Doppler shift representing the difference signal could be increased, seismic signals having smaller amplitudes could be detected.